Award: OCE-1232986

The guiding mission for the international GEOTRACES program is to “identify processes and quantify fluxes that control the distributions of key trace elements and isotopes (TEIs) in the ocean” (GEOTRACES, 2006).  Particles are a key parameter for the GEOTRACES program because of their role in the cycling of so many trace elements.  The U.S. GEOTRACES East Pacific Zonal Transect between Peru and Tahiti was designed in a community effort to cross three biogeochemically important settings: large lateral gradients in productivity, a major oxygen deficient zone off the coast of Peru, and the world’s most prominent hydrothermal plume emanating from the southern East Pacific Rise.  We were funded to collect size-fractionated particles by in-situ filtration on the GEOTRACES East Pacific Zonal Transect to measure their major phases and trace element compositions. 

The University of Minnesota group was specifically tasked with measuring the chemical form of iron in the world’s most prominent hydrothermal plume emanating from the southern East Pacific Rise mid-ocean ridge spreading center.  This is an important research endeavor because hydrothermal vents introduce a lot of iron to the deep ocean, but scientists do not know how much of this iron makes its way to iron-deprived locations in the ocean.  The study was challenging to design and undertake because the plume particles are very small in size, often few in number, and composed of complex mixtures of biological, organic, and inorganic materials.  To overcome these challenges we used analytical tools that could overcome and address each of these difficulties.  Specifically, we used synchrotron radiation X-ray microprobe and microscope instruments available at the Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, CA.      

 

We hypothesized that molecules containing carbon- and sulfur- would slow iron oxidation within the hydrothermal plume. This hypothesis was based on recent observations at the northern East Pacific Rise (e.g. 9^oN), as well as historical plume studies indicating abundant sulfur along the southern East Pacific Rise (15^oS) segment of the mid-ocean ridge. In contrast to our hypothesis, the 15^oS plume and underlying sediments were depleted in both particulate sulfur and chemically reduced iron. Essentially all iron detected was in the form of an oxidized and poorly soluble for (i.e. iron(III)).  Although sulfur and sulfur-bearing minerals (such as pyrite, FeS₂) were undetectable in all of our plume particles (greater than 0.2 micrometer in di...